text:
Are there any LSA aircraft that can be flown IFR in IMC?
Are there any LSA aircraft that are IFR certified? A LSA would be the perfect private commuter plane for an instrument rated private pilot.
If not, what are the most cost effective airplanes that are IFR certified? Is there anything cheaper than a C172?
Just to be clear: I am asking about aircraft that are IFR certified, meaning that they can be flown IFR in IMC conditions. I am also aware that there is a huge difference between different countries with regard to aircraft certification so my question is mainly about the U.S.

Are you asking about the US specifically?

Question updated. Although it would be interesting to know if there are any light aircraft that can be used in IMC in Europe.

I question the premise that an LSA would be a perfect commuter plane.  Their cruise speed is limited by regulation.  There are plenty of experimental amateur-built aircraft that can legally and practically be flown IFR, and many are cheaper than a C172.  My VariEze cost me $25k including upgrades to a WAAS GPS, and I commuted with it four times a week for several months.

Well, if the question were simply "Are there any lightsport aircraft that can be flown in IMC", the answer would certainly be "yes".  Don't ask me how I know.

@quietflyer That's exactly what the question is, literally, and your answer is extremely helpful...

Sorry to reanimate this rather old question, but I'm surprised none of the previous answers mentioned the Evector SportStar MAX, which has a model that is IFR equipped and which was available several years before this question was asked. From its POH (section 1.2):

Certification basis
SportStar MAX complies with the ASTM F2245-07a Standard Specification for Design and Performance of a Light Sport Airplane, issued by ASTM International Committee F37.
IFR version complies with FAR 91.205 requirements, ...

Here's a Plane & Pilot review of the IFR version, and here's a Press Release from Evector about this model.
Evektor followed the MAX model with the Harmony, and at least one place on their website suggests it too is available in an IFR equipped version.

All three of the previous answers specifically call out the difference between an IFR-capable aircraft and IMC conditions, and note that currently IFR capable LSA are not permitted to fly in IMC conditions, which was the OP's question.

@FreeMan My question was about LSA aircraft that were certified for IFR in IMC. This LSA (SportStar MAX) is indeed certified for that (quote: "The MAX IFR is the first Light Sport Aircraft that is Part 23 instrument compliant for IFR IMC flight". Unless I'm missing something, this answer is the only one that specifically answers the question.

Ah, @PhilippeLeybaert, I missed seeing that. My bad.

A quick websearch leads me to believe that you can fly a properly equipped LSA on an IFR flight plan (eg: being directed by ATC), but you cannot fly an LSA into IMC.IMC and IFR are not the same thing, and LSA's may be equipped for IFR flight, but they cannot be certified for IMC flight.
Sources:

LSA for Instrument Training?
Light Sport Aircraft

More info, April, 2017:
https://generalaviationnews.com/2017/04/17/can-you-fly-ifr-in-an-lsa/

In summary, if you are an instrument pilot, and if you are current, and if you have a medical, and if you purchase an aircraft like the Bristell and register it as an ELSA, no regulation prevents you from filing and flying IFR including into IMC.

I meant IFR in IMC. I will update my question

It also appears that a number of articles I find online totally confuse and blur the lines between an IFR flight in VMC conditions, and an IFR flight in IMC conditions.  Those are big, important differences!

There is at least one LSA that is approved for IFR flight: The Zodiac 650 (available as a kit, or a factory-built Special Light Sport Aircraft).

More broadly, you can operate light sport aircraft under IFR (in VMC or IMC) as long as it's permitted by the operating instructions. AOPA's Light Sport / Sport Pilot FAQ addresses this well, so I'll just steal their answer:

Can I fly a special light sport aircraft (SLSA) in IFR conditions or at night?
Only day/VFR conditions are specifically addressed in the ASTM consensus standards that govern the production of SLSA. Being that sport pilots and those exercising sport pilot privileges are limited to flying only in day/VFR conditions, this seems appropriate.
On the other hand, if an appropriately rated pilot (example: private pilot with an instrument rating) wants to fly SLSA under IFR or at night, the aircraft's operating limitations must allow it, and the aircraft must be equipped per 91.205 for VFR flight at night and/or IFR flight. Additionally, 91.327(d) requires all SLSA to be operated in accordance with the aircraft's operating instructions. Operating instructions differ from operating limitations in that the engine, airframe, and accessory manufacturers issue them; the FAA issues operating limitations.
An example of operating instructions is a SLSA equipped with a Rotax engine. Rotax's operating instructions prohibit the use of a Rotax engine at night or in IFR conditions unless it is the FAA type certificated engine (14 CFR part 33). Other engine, airframe, and accessory manufacturers might impose similar restrictions.
If you are appropriately rated and would like to operate a special light sport aircraft at night or under IFR, contact the manufacturer to determine if any provisions can be made.

(Note also that there were some rumors of the ASTM consensus standards changing at some point to require DAY VFR ONLY placards on new Special Light-Sport Aircraft -- At this time I don't believe that's happened yet, but it might...)

Yes. As long as the pilot is IFR rated with a medical certificate and the aircraft is properly equipped. Including IFR certified lighting, avionics and power-plant.
The ASTM commitee voted to ban LSA flight into IMC, but that has not been approved by the FAA, as mentioned here.

Yes. Bristell has provided a legal path for this in the US. The aircraft must be certificated as an ELSA, not an SLSA, and all the other IFR requirements must be met. https://generalaviationnews.com/2017/04/17/can-you-fly-ifr-in-an-lsa/


text:
How much drag do the gear doors create during retraction?
It's been said that if you retract the gear at a critical moment that performance will actually be worse than if you left the gear down. However, is this really true, and just how long is the performance worse for? Obviously in the long run it'd be better to have the gear up, so how much drag is there actually?

Of course gear doors create drag when open.  The question is "how much?"
As you suspect, door drag is not the critical factor here.  As usual the answer to your question is aircraft-specific, and your question is pointing to a generalized "old wive's tale" which is only true sometimes.
OK, what is the story behind the story?  Look at this Youtube video.  Watch the main gear as it enters/exits the storage wells in the fuselage.  The axles of the wheels are close to parallel to the direction of flight, which means the wheels are being dragged broadside in the slipstream.  A very high drag scenario which will affect aircraft performance.  The POH recommendation is (from memory, now) to climb over critical obstacles on takeoff with the gear down, and raise the gear only when the obstacle is cleared.  The attached video shows why.
This advice does not pertain to all planes.  Know your plane, read the POH.  Oh... and ask questions here!

Just curious - what about interference drag increasing during gear transition is an old wives' tale?

Actually, the doors have to create more drag when they are open -vs- closed and flush with the wing.  I don't see how this could be an old wives tale either (other than possibly the need to wait to retract the gear?).

This is not about the doors.  Certain planes like the Cessna 210 have a lot more gear drag during gear in transition than gear down or gear up.  Somehow this gets translated that all planes have this high drag scenario with gear in motion.  While true for some planes like the 210, it is certainly not a universal situation - and that is the old wives tale.  Look at the video in this thread to see why.

That's a good explanation, and it would be great if you could incorporate it into your original answer - I'd love to see your answer made slightly less hand-wavy and a little more explanatory. I love making students think through the full answer, but I'm not sure it's an effective strategy here.

It depends on the airplane, of course, but opening the gear doors will cause more drag just as anything sticking out into the wind stream will.  As a pilot you don't need to know "how much" drag is created because the manufacturer has already developed the performance numbers and procedures for you to use.
These numbers are developed by actually flying the airplane and will include the performance loss due to the movement of the gear/gear doors.  If the manufacturer finds that the performance loss is too great, they will create an operational pilot procedure that requires the gear to remain down.  Otherwise, long term, it is better for climb performance if it can be raised.
Part 23 commuter and Part 25 aircraft have specific climb gradients that must be maintained even with an engine loss, the gear down, and the gear doors open (normally the worst case scenario).  The manufacturer must ensure that the aircraft can meet this performance requirement somehow (sufficient engine power, reducing maximum allowable takeoff weight, etc.) and as long as you follow the Flight Manual requirements you will get the best possible performance from the airplane.
They tried every combination (and told you about the best) so that you don't have to be a test pilot and start experimenting when something goes wrong.  :)

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
How does the mixture of an engine affect the engine's operation?
I know that in most piston aircraft that there is a mixture control for the engine. My question is, how does the mixture control affect the engine's operation and what type of situations dictate a particular mixture setting? Would an incorrect mixture setting be detrimental, a minor effect or nothing at all?

Simple Answer: Running the engine lean or rich at different altitudes.
Longer Answer: There are a couple of reasons you want to adjust the mixture of a aircraft engine.

Since air density changes with altitude, it's a good idea to be able to change the fuel/air mixture at different altitudes.  If you did not adjust the mixture at high altitudes you may not get full power because the air is thinner and now there is same amount of fuel being mixed with less air.  All the unburnt fuel can cause issues like loss of power and/or fouling of plugs.
Being able to lean the mixture at higher altitude lets you save fuel, so you can go further on a full tank.  (hence saving money)
Running a engine rich can help cool engine.  Fuel acts like a coolant in this manner.  Typically on climbing you want to run the engine a bit rich because your going slower and less air is able to help cool the engine.
Traditional piston powered aircraft engines are shut off by pulling the mixture to the off position.  This starves the engine and burns off remaining fuel in the cylinders.

This topic can also bring up other discussions.  Like EGT, CHT, running an engine rich of peak or lean of peak, monitoring fuel flow rate with a flow meter, and fuel injectors on fuel injected systems.
Hope this helps.

The mixture control allows you to increase (to run "richer") or decrease (to run "leaner") the amount of fuel in the fuel-air mixture. As a general rule, you want to "lean out" the engine at higher altitudes because the air is thinner, and it takes more air (by volume) to burn the same amount of fuel.
Mixture controls are used with many piston engines. However, turbine engines and some piston engines with fuel injection or altitude compensating carburetors do not have mixture controls.
In addition to zingle-dingle's points, it is important to lean an engine for takeoff at high elevations. At a 7,000 ft (or higher) runway, for example, an engine with a full-rich mixture setting will not generate nearly as much power as one properly leaned out.
Since the normal mixture setting at low-elevation airports may be full-rich, this may come as a surprise to someone visiting the mountains. It's not uncommon for a plane to crash at a high elevation airport because the engine was not leaned properly for takeoff.
Conversely, if an engine is properly leaned out at 14,000 ft., for example, the mixture should be adjusted to be more rich on or before a descent to 1,000 ft. to prevent engine overheating or possible stoppage due to too little fuel in the fuel/air mixture.

The stoichiometric mixture is the ideal chemical ratio of air:fuel, that will burn all the fuel in the mixture with no excess oxygen left. It is located at somewhere around 15 parts air and 1 part fuel per unit of mass. Mind you: the ideal ratio is based on mass, not on volume so if air density changes (as you climb/descend), as the carburetor is basically mixing volumes, your air:fuel ratio will change with altitude.
Mixture control will help you adjust that ideal 15:1 ratio, between what is called "lean" (more than 15 parts air), and "rich" (less than 15 parts).  Most power is available at the ideal ~15:1 so normally that is where you would want to aim your mixture. However:

Running a bit on the "rich" side of ideal will consume more fuel,
generate less power, and might cause carbon deposits, but the
extra vapor-fuel that remains unburned will serve as a coolant and reduce
the temperature of your cylinders. That is why most of the times the
pilots will set mixture on the "rich" side of ideal.
Running a bit on the "lean" side of the ideal will consume less fuel,
will generate a little bit less power, but the lack of cooling from
the extra fuel will eventually overheat the cylinders, leading to all
sort of nasty stuff like detonation and overheating the oil.

As for the particular situations calling for operations: 99% of the times you simply want a good/ideal mixture, which is why mixture control is pretty much getting automated
these days. There are very few exceptions I can think of: for long cruise at cold temperatures I might try a leaner mixture to see if I can skim a couple of pounds of fuel. Or in an emergency engine-coughing situation I might cram the mixture to full rich to prevent the engine from starving or overheating. But then again, if mixture is automated the FADEC will do that anyway.

This was all good until the section on lean of peak operation. If managed correctly with fuel injection it not only doesn't result in engine damage but normally will prolong engine life. However, it does require careful management to ensure A) the mixture is lean enough as running near the stoichiometric ratio will result in overheating and damage and B) no individual cylinder is operating outside the desired range, which requires a decent EGT monitor and well adjusted jets. Also in private GA aircraft very few are FADEC, most still have manual mixture controls due to cost.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why is "dragging it in" considered bad in small aircraft, but fine in larger aircraft?
In larger aircraft, shallow, high-power approaches are performed as a matter of routine, but in light aircraft it seems to be frowned upon, especially in clear weather and smooth fields. Why is "dragging it in" considered bad in small aircraft?

I can't say this for sure, but I suspect it has something to do with the amount of time for a turbine engine to go from low to high power versus a piston engine to go from low to high power.  Safer to keep some power in, just in case.

For the GA fleet, there is some historical precedent operating here.  Years ago, engines were much less reliable than they are today.  Dragging it in generally means that you cannot glide it in if you have a complete power failure.  This translates to saying in the event of an engine failure, you are landing off airport with limited selection of landing sites (your altitude is low as you are now below pattern altitude).  So if you are flying behind an old Curtis OX-5 in your Jenny, a gliding approach to landing was recommended back in the day.
Engines are much more reliable today.  After an analysis of landing accidents, the FAA determined that a stabilized approach to landing is less likely to result in an accident, and they changed their guidance accordingly. Stabilized approaches are partial power-on landings.  Some old timers and some old time CFIs reject the FAA's statistical analysis and still teach/fly glide-it-in landings.  Properly executed, both approaches to landing are safe.  It is just that the FAA thinks that stabilized approaches are safer given engines commonly in use today.
It is also true that virtually all airlines require stabilized approaches in their operating specs, and that a go around is mandatory if the approach is not stabilized.  With this in mind, flight schools that aspire to send their graduates to the majors will all teach stabilized approaches.

This is less of a "big -vs- small" question, and more of a "single-engine -vs- multi-engine" question.
So, to answer your question with another question: What would happen if an engine were to fail during the approach in each case?
In a single-engine aircraft you want to always be in a position to land if the engine fails, so you don't want to get too far from the runway, while in a multi-engine aircraft you can continue the approach if an engine fails.
You want to be careful not to take this concept too far though.
See:  Short approach vs Normal
Also, in jet aircraft the power needs to be kept relatively high so that additional power is available quickly if needed since the engines take awhile to accelerate.
See:  Why do turbine engines take so long to spool up?
Partially because of this, the idea of a stabilized approach was widely incorporated.  This requires that the aircraft is in a stable, continuous rate descent to the runway and that the approach be abandoned if not within required parameters at a specified minimum altitude.
A secondary reason is that "larger" aircraft are commonly flying instrument approaches to help transition from the enroute/terminal phase of flight to the runway.  These standard procedures are designed for use in instrument conditions and allow plenty of time to safely configure for a stabilized landing.  Even in visual conditions, they are commonly used when there is a lot of traffic to "put everybody in line".  Even a small single-engine airplane will probably be further from the runway than they like at big airports, just because of the amount of other traffic.

Actually a power on stabilized approach makes sense. Being close the runway in case power fails does not. Else you would never leave the pattern! After a cross country wherein my engine has been running just fine, why would it all of a sudden be an issue when I land? Now if you do have issues, carb ice, oil tempt, etc, I understand the safety margin. Also it is Good for “off field” engine out practice. So I do both, in the Citabria I glide it in from downwind abreast the numbers. Skyhawk SP I do a power assisted approach. Mostly because that is what you do in IFR flying, it’s good practice even when vfr. That is the main reason big guys power it in. Same procedure every time regardless of external conditions builds consistency and kiss! Also turbine engines are extremely reliable! Remember, no matter what you choose, a stabilized approach after a stable pattern is the foundation of good to great landings. I am not a cfi, so this is merely my experience not instruction!


text:
Is IAS without instrument error the same as CAS?
I was reading a POH which stated something along the lines of

NOTE
Except as noted, all airspeeds quoted in this handbook are Indicated
  Airspeeds (IAS) in Knots and assume zero instrument error.

Why didn't they denote that as CAS (calibrated airspeed) instead, which is without instrument error? Is there a key difference between the two that I'm missing?

Even without instrument error, IAS does not equal CAS. This because there are two kinds of errors that make up the difference between Indicated Airspeed (IAS) and Calibrated Airspeed (CAS)

Installation error
Instrument error

Installation error is basically a pressure error. Airspeed measurement is a measure of dynamic pressure of the undisturbed airflow. When the pitot tube is not placed directly into undisturbed airflow the IAS will not equal the CAS. Another source of installation error are leaks in the pitot system.
Instrument error is the error introduced by ASI itself by transforming the pressure to a speed indication. For example calibration errors of the pressure sensor or excessive friction in the mechanical drive system of the dial fall into this category.
The POH in your example assumes no Instrument error, but is corrected for installation error. An implicit assumption is made that there are no leaks in the system, so the POH corrects for airflow disturbances. That is why some aircraft have different tables for different configurations; the configurations affect the airflow around the pitot tube.
In aircraft equipped with electronic Air Data Computers (ADC's) the installation error is sometimes automatically corrected by making use of angle of attack sensors, electronic configuration information and lookup tables.

Instrument error is usually something to do with the actual instrument (like the arm on the dial or something). The primary difference between IAS and CAS as I understand it is air pressure discrepancies in the Pitot-tube due to different angle of attacks and airflow disturbances due to the airframe. The POH of the Katana DA20 I regularly fly has a chart mapping IAS/CAS, with two different lines corresponding to different flaps configurations.
I'm no aeronautical engineer and this is a layman's understanding, so feel free to correct me. As I understand it, IAS (without instrument error) is the measured air pressure, CAS is the theoretical air pressure which would've have been measured, would it be possible to measure it perfectly, and instrument error is due to machining imprecisions or otherwise errors in the construction of the instrument.
EDIT
Apparently the question wasn't if IAS without instrument error is the same as CAS, but why not use CAS throughout the POH. The easy answer is, since the CAS cannot be measured, but IAS can (to the extent that construction precision allow) and is displayed to the pilot, why should he memorize numbers which he'll never see? Charting IAS makes much more sense, and although they'll be offset by instrument errors, those are (generally) individual to the aircraft, and so it's the closest you're going to get in a printed manual which is not a one-of copy for a single aircraft.

Yes, I understand that much... What I was asking for was if IAS assumed to have "zero instrument error" is the same as CAS. :)

@Qantas94Heavy; did i get your question right this time?

@Qantas94Heavy; ok, cool, I thought it was the 'difference betwen IAS and CAS'-part (in the title) you were interested in; maybe it was just me, but the question seems a bit unclear.

I've edited the title -- is that better?

@Qantas94Heavy; You've got me confused. :) Still sounds like you're asking for the difference between IAS and CAS (which I - at least attempted - answered originally)

Well the difference between the two is a part of the question, because without that you wouldn't be able to answer which part of it is the main issue, could you?

@Qantas94Heavy; yeah, you still got me confused.. ;) But if you're happy with the answer as it stands, I'm happy. :)

Okay, doubly confused you, I'll try again. My question was "Why not use CAS throughout the POH, if IAS without instrument error is the same as CAS?", but the part about "if IAS without instrument error is the same as CAS" was crucial -- as you pointed out, it isn't the same, which is why I put "Is there a key difference between the two that I'm missing", because I thought that there had to be something more to this assumption than the obvious.


text:
When cleared for an approach and not on vectors, is a depicted course reversal always required?
At least where I fly, most instrument approaches now use a Hold In Lieu of Procedure Turn (HILO). The "full procedure" now consists of a turn in the hold before proceeding inbound.

In the example above (click to embiggen), if a pilot is northeast of EYWOK and receives the following clearance from Seattle Approach:

Cessna 123AB, proceed direct EYWOK, maintain 3000 until established, cleared for the ILS Yankee approach at Paine Field.

Are they required to fly the course reversal, or can they proceed inbound on the final approach course immediately upon reaching EYWOK and fly direct to ITIPE?  If not, what instruction must be received in order to skip the HILO procedure?

This is the sort of thing one might see on an instrument checkride, albeit as a warmup softball sorta thing.

Yes, with this procedure you are required to fly the procedure turn unless you are flying the CVV transition (which is notated as NoPT), are receiving vectors to final, or receive a clearance for a "straight-in approach" over EYWOK.  If you are cleared direct to the final approach fix, none of these apply (but remember that you can always request a straight-in approach if you don't want to do the course reversal)!
FAR 91.175

(j) Limitation on procedure turns. In the case of a radar vector to a
  final approach course or fix, a timed approach from a holding fix, or
  an approach for which the procedure specifies "No PT," no pilot may
  make a procedure turn unless cleared to do so by ATC.

The AIM states:

5-4-6. Approach Clearance
4. If proceeding to an IAF with a published course reversal (procedure turn or holdinlieu of PT pattern), except when cleared for a
  straight in approach by ATC, the pilot must execute the procedure
  turn/holdinlieu of PT, and complete the approach.
5. If cleared to an IAF/IF via a NoPT route, or no procedure turn/holdinlieu of PT is published, continue with the published
  approach.
6. In addition to the above, RNAV aircraft may be issued a clearance direct to the IAF/IF at intercept angles not greater than 90 degrees
  for both conventional and RNAV instrument approaches. Controllers may
  issue a heading or a course direct to a fix between the IF and FAF at
  intercept angles not greater than 30 degrees for both conventional and
  RNAV instrument approaches. In all cases, controllers will assign
  altitudes that ensure obstacle clearance and will permit a normal
  descent to the FAF. When clearing aircraft direct to the IF, ATC will
  radar monitor the aircraft until the IF and will advise the pilot to
  expect clearance direct to the IF at least 5 miles from the fix. ATC
  must issue a straightin approach clearance when clearing an aircraft
  direct to an IAF/IF with a procedure turn or hold-in-lieu of a
  procedure turn, and ATC does not want the aircraft to execute the
  course reversal.
7. RNAV aircraft may be issued a clearance direct to the FAF that is also charted as an IAF, in which case the pilot is expected to execute
  the depicted procedure turn or holdinlieu of procedure turn.  ATC
  will not issue a straightin approach clearance.  If the pilot desires
  a straightin approach, they must request vectors to the final
  approach course outside of the FAF or fly a published “NoPT” route.
  When visual approaches are in use, ATC may clear an aircraft direct to
  the FAF.

I realize that this is a pretty old post, but I stumbled upon it and am puzzled by what the AIM says, as well as your interpretation.  Let’s simplify the scenario further and presume that our aircraft is on the 342 radial, heading inbound on a heading of 162.  If cleared for the approach, but not specifically cleared for a straight-in, your position seems to be that the aircraft should execute a turn in holding, is that correct?

And if "straight-in" is not heard the pilot should query ATC?  Because a course reversal is clearly not required, and without being cleared into holding I can't imagine saying nothing and doing an unnecessary lap.  But if asking "understand cleared for the straight-in?" I could imagine a snarky "weeellll... would you like to hold?" kind of a response.

@michaelhall One thing to keep in mind is that a course reversal is not always used to align yourself with the final approach course. It can also be used as a means to descend to an altitude on the approach while assuring obstacle clearance.  In your example, yes you are expected to perform the course reversal, if you have been cleared direct to the FAF (you didn't specify that).  Paragraph 4 if the AIM is very specific, and applies directly to that situation.  As far as asking, if you ever aren't sure what they want, please DO!  I'll take a snarky response over a violation, or worse.

Thanks for the answer.  Most of my time is military, and I can't say that I have ever encountered this specific scenario.  I totally agree with asking if unsure, and I have asked many times for an offset, or one turn in holding for alignment.  In my scenario though, cleared to the airport (vs IAF) and cleared for the approach, even lacking the specific words "straight-in" from the controller it would never occur to me that a lap in the hold might be expected.  If a turn was needed for spacing or descent I would expect the controller to tell me.  I need to watch for this now!  Thanks again.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is a monitored approach?
I've heard this term used to describe approaches into low weather (e.g. category 2 ILS), what is it?  How is this different than an ILS approach?

A monitored approach is a special kind of instrument approach involving added verbal call outs and increased monitoring of the airplane and is typically conducted when weather is below a certain threshold. For example, a crew may be required to fly a monitored approach if the weather is below 3/4 mile vis and the captain has less than 100 hours in type, or in any case with the weather below ILS Category I minimums. It is also specific to multi-crew aircraft and requires at least 2 pilots and in many cases an autopilot.
In a crew environment, the pilots are typically designated as Pilot Flying (PF) and Pilot Not Flying (PNF).  In general the PF is flying and making commands of the PNF, and the PNF mans the radios and operates gear and flaps the command of the PF.  More detail of this setup probably deserves its own question.
To illustrate a monitored approach, I'll describe on based on my companies procedures.  The specific requirements can vary between operators based on crew training, aircraft equipment and approved opspecs.
The monitored approach briefing
We have identified the arrival airport current weather is 1200 RVR and obscured ceilings and we plan to fly the ILS to category II minimums.  Myself and my first officer have both have current category 2 training, neither of us are high-minimums, and the aircraft has no MEL items effecting category 2 operations.  We are legal to fly the approach, but company operations require we fly it as a monitored approach.
Regardless of who has been PF and PNF up to this point, the first officer becomes the PF and the captain the PNF.  The PF briefs the approach as normal except for:

CAT II radar altitude minimums
Low vis taxi routes if applicable
The approach will be flown by the autopilot (company specific)
Special autopilot considerations
Required verbal call outs (see below)

The monitored approach
For my company, the autopilot had to be engaged in "CAT 2" mode before glideslope intercept.  This will vary by aircraft and company.  For the approach, as noted earlier, the first officer is PF, captain is PNF.
The PF duties are to monitor the autopilot and to execute the missed approach if the autopilot has problems.  The PNF also monitors the autopilot.  The pilots are monitoring the autopilot for performance and for indications it cannot meet required tolerances for the approach.
PF will call for gear and flaps as normal, except a non standard flap setting may be used for the approach (e.g. we flew flaps 22 instead of 45).
Once configured for landing, the PF will call for "Landing checklist - to the line" (company specific), this for us was the normal landing checklist with the caveat that the checklist is complete when the captain announces "Landing, I have the airplane" (more on this in a moment).
Nearing the Cat II decision height (typically 100 ft RA), the PNF will announce "Going heads up", discontinue monitoring the autopilot and look for the approach lighting system and the runway lights.
The PF will continue to monitor the autopilot.
The airplane will announce "Minimums" at the radar altitude DH and when the airplane reaches this height, the PNF will fly the missed approach.
If the captain sees the required lighting to continue the approach, they will announce "Landing, I have the airplane", and the FO will reply "you have the airplane, landing checklist complete.
The Captain must disconnect the autopilot (by 50 feet, no autoland for us), and would fly any aborted landing executed after taking control.
Caveats
Rules on autopilot usage, disconnect heights and autolanding will vary based on aircraft and company policies.  My example assumes an Embraer-145 certified for Cat 2 approaches and no autothrottles or autoland capability.  Checklist verbiage and flap configurations mentioned also would vary by aircraft and company.  And finally, the specific callouts will vary by company.
While it seems like there was a lot of Category II specific things in my example, the two concepts of reduced visibility ILS approaches and monitored approaches are intertwined, and you will almost always be flying a monitored approach because it is required to conduct the low weather approach by company policy.

Terminology
"Monitored approach" is the most commonly used term for a specific allocation of pilot tasks during an approach to land. It involves a different distribution of workload to the traditional one, in which a single individual (the "pilot") is assigned all the aircraft handling tasks for instrument approach, visual landing and instrument go-around phases, as well as decision-making and overall management of the flight. The other individual - the "co-pilot" -is assigned the Pilot Monitoring tasks throughout.
In a "monitored approach", pilot duties are divided differently. One individual is responsible for overall management, decision-making, monitoring of all instrument flight during approach and go-around, and handling during the visual landing phase.
The other's responsibilities are aircraft handling during instrument flight (approach and go-around), and monitoring during visual flight.
Thus, the "co-pilot" controls the aircraft on approach exclusively ‘head down’ by reference to the flight instruments, whilst the "pilot" monitors and oversees the flight prior to either (a) assuming control for landing, or (b) continuing to monitor and manage in the event of a go-around being necessary.
This basic duty allocation has also been known by other terminology including but not limited to "delegated" , "pilot monitored" "split", "shared", or "control handover" approach.
A significant reason for these terminology variants is to accommodate concerns that limiting the application of the term "monitored approach" exclusively to this duty allocation implies that other duty allocations do not require the approach to be monitored, when there are clear obligations to do so.
Similarly, the term has occasionally been applied to a totally different concept, when the aircraft's approach is monitored by air traffic controllers on the ground using radar, in a radar-monitored approach or ground-monitored approach.
Consequently there has been ambiguity in the flight operations community about what is meant when the term "monitored approach" is used (see George Mason University study for FAA, 2004, page 7: "Ambiguity of the term").
For clarity, in the description and arguments sections here the term "Pilot-in-charge Monitored Approach" will be used for this purpose under the acronym "PicMA", as it specifies that the pilot in charge and responsible for the major flight decisions is performing the Pilot Monitoring tasks during the approach. Use of this term will encompass all known specific implementations of the procedure.
In the "history" section the term "monitored approach" in quotes will be used as it represents the terminology used in most references.
The abbreviation "P1" will be used for the "Pilot in charge", who is normally the pilot who has made the takeoff and is anticipating making the landing. "P2" means the other, second pilot or co-pilot.
Description
The following description is of the most widely applicable implementation. Individual operators may use more restricted variants, as will be noted later.
In "traditional" operations, it is normal for the Aircraft Commander to decide which pilot will undertake the "Pilot Flying" (PF) tasks and which the "Pilot Monitoring" (PM) tasks, in accordance with company-specific criteria. Typically this will then apply throughout the flight. In operations using PicMA, the same decision simply determines which pilot will undertake the "Pilot in charge" (P1) role and which the co-pilot (P2) role.
Prior to the commencement of the descent and approach, the crew perform the required approach and landing briefings to achieve a common understanding of the intended plan. For example the P2 may present a plan for the descent, approach and go-around, for acceptance or modification as appropriate by the P1. The P1 outlines intentions in relation to the landing conditions etc.
The P2 then assumes the Pilot Flying tasks, and the P1 the Pilot Monitoring tasks, including normal communications with Air Traffic Control. The P1 role includes overall management of the flight as Pilot in charge. During the approach all normal operator-specified callout tasks are made as and when appropriate.
Approaching the applicable Decision Altitude/Height (DA/DH) the P1 (monitoring) pilot starts seeking external visual references, with the objective of being in a position to announce a prompt decision at DH with an unequivocal call of either “Land” or "Go-Around".
At DH the P2, as "head-down" pilot, calls “Decide”. If the P1's response is “Go Around”, the P2, head-down pilot flies the prescribed or ad-hoc cleared missed approach, and the P1 continues in the pilot monitoring and management role.
If the response to the "Decide" call at DH is “Land”, the P1, who previously been monitoring and has made that call, takes over as PF and continues to the landing. The P2 relinquishes control and continues with the monitoring tasks, with attention primarily devoted to continued monitoring of the instruments.
If visual reference should subsequently be lost below DH, or other circumstances call for a missed approach after the P1 has taken control - a baulked landing - then the P1 calls for and commences the go around. The P1 subsequently hands back control to the P2 a suitable point during the missed approach climb out, to consider options for completing the flight such as either another approach or a diversion.
For non-precision approaches, a "Continuous Descent Final Approach" with a Decision Height is normally recommended as safer than a "step-down" or "dive and drive" with a Minimum Descent Altitude/Height (MDA/MDH) and a Missed Approach Point. However PicMA procedures can be used for step-down procedures using an MDA/H, using a call by P2 of "maintaining minimum" (or equivalent) on reaching the MDA/H, with the Decide call being made at the Missed Approach Point. In this case the level portion of a step-down procedure will be flown more accurately and safely.
If all conditions for a safe landing laid down by the operator have been achieved well above DH, i.e. "stable approach criteria" are met and full visual reference has been established, the P1 may assume control at his/her discretion with a declaration of "visual, I have control" or equivalent. Depending on operator preferences, the P2 will be expected to continue instrument monitoring and make appropriate callouts.
Variations
Detailed ICAO Standards and Recommended Practices (SARPS) do not exist for crew cockpit duty allocations, hence significant variations can be found. The PicMA procedure as described can be applied to all categories of operation in both IMC and VMC, and all types of instrument approach.
The PicMA procedure concept is fully compatible with "leg and leg flying" and "role reversal", and does not impede the ability of First Officers to obtain either handling experience or experience for licensing purposes as "Pilot in Command Under Supervision".
Many operators use specific weather criteria to determine when to use a PicMA procedure. Typically in these cases the flight has started with the crew using "traditional" PF/PM duty allocations, and a decision to change to a PicMA procedure is made by the Captain on the basis of weather information received en route. Such weather criteria are generally based on reported or forecast visibility and cloud cover, and the degree of discretion afforded to the pilot in command in respect specific types of approach and/or prevailing weather conditions may also vary from operator to operator.
In North America, such criteria have tended to involve crew rank as a factor. It is often allied with the prevalence of conditions in which the First Officer is not normally authorised to make a landing. In such cases it may be mandatory for the First Officer to be the P2 to fly the approach/go around, and the pilot in command as the P1 to monitor the approach/land. This limitation is less often the case in Europe and elsewhere.
For Low Visibility Operations, authorities may approve variations in the point at which the pilot role change takes place depending on category of operation, e.g. Category 3 operations where an automatic landing is mandatory, or enhanced low visibility equipment such as Head Up Displays are required. Operators may also vary the degree of discretion afforded to the pilot in command in respect of using the method for specific types of approach and/or prevailing weather conditions.
Comparisons between "Traditional" and PicMA procedures
Air transport operations predominantly use approach procedures with no exchange of PF tasks. Aircraft manufacturers generally provide task-based operating procedures for individual aircraft types, which describe principally tasks for Pilot Flying and Pilot Not Flying/Pilot Monitoring. These are often then translated into Operators' own approved manuals with little modification.
As PicMA involves exchanging the tasks of PF and PM between pilots, it is often interpreted as being a significant departure from the manufacturers' recommendations, although in fact they may be silent or equivocal on the subject, or even support the use of PicMA in line operation. As a result, safety-based arguments can be found both for using PicMA procedures and for maintaining the status quo.
Although frequently thought to be only a "bad weather transition" procedure, the principal arguments in favour of using PicMA revolve around three main consequences of the redistribution of workload. These address the most common types of contributory causes in so-called "crew-caused" approach and landing accidents:

inadequate planning and preparation for the conditions actually encountered;
ineffectiveness of the monitoring of the Pilot Flying by the Pilot Monitoring
inappropriate or premature attempts to transition from instrument to visual cues.

All these are extensively described in accident reports and have been the subject of large amounts of research, examples of which are given in the "history" section.
Planning and preparation. It is contended that when using PicMA procedures, approach management is improved by the PiC's delegation of detailed handling tasks. Starting with better crew interaction and teamwork in descent and approach planning, the P1's capability to maintain situational awareness is enhanced, and mitigates problems of communications and interpretation between crew and external agencies.
A lower routine workload provides more reserve capability for threat and error management. Having the flying assigned to the P2 on the approach, and with the P2 mentally preparing for a go-around not a landing, the risk of "plan continuation bias" leading to a dangerous landing attempt is reduced, as is the "startle factor" if a go-around is necessary.
Ineffective monitoring. For the second type of factor, it is contended that in all conditions, transferring the main monitoring task to the pilot actually in overall charge of the flight increases monitoring effectiveness. This is attributed to its reversal of the normal cross-cockpit authority gradient, and procedurally eliminating the "First Officer's dilemma" - whether to continue to assist the Captain in a course of action the F/O believes may be unwise, or actively oppose it.
It addresses commonly recognised cultural issues in traditional monitoring procedures by promoting monitoring as a fundamental active command role, which builds on strengths and reinforces weaknesses in both "authoritarian" and "individualistic" cultures. Accepting that "monitoring" means "supervision" and associating monitoring with accountability and authority encourages a conservative mind-set and discourages risk-taking.
Overall it is contended that this significantly reduces monitoring / challenging errors and provides better protection against "tactical errors" and "errors of omission".
Transition to visual cues.  In the third type of contributory cause, unsafe  transition from instrument information to external visual cues, especially in bad weather when approaching Decision Height / MDA, overall command capability is enhanced as the P1's situational awareness is better maintained through having to concentrate less on detail handling tasks. The maximum possible time is provided for assessment of developing visual cues, while premature transfer to visual cues which can be illusory is minimised.
There is more assurance of a positive decision being made at Decision Height, rather than below it. Unbroken monitoring of instruments can be assured as both pilots are not exposed to looking out at the same time. This enhances the detection of low altitude flight path deviations in a continued approach to landing, and the PM's callout reliability is also enhanced.
In a Go-around from DH, the PF is already fully attuned to instrument flying and "primed" for the go-around. Extensive references supporting these contentions have been collected and can be found at www.picma.org.uk.
By comparison, arguments are also made for maintaining the status quo. These fall into a number of areas - actual safety risks incurred, technical redundancy / irrelevance, and pilot satisfaction / conservatism. For the most part they are contentions whose proponents have not supplied supporting arguments or evidence, beyond the fact that the changes to the status quo are not generally supported.
It is contended that a low level exchange of control is inherently hazardous, due to factors such as

a need for the P1 to get the feel of the controls,
a need for the P1 to deal with out-of-trim conditions or unstable flight path conditions that have been accepted by the P2,
risk of confusion as to who has control resulting in one pilot trying to go around while the other tries to land, and
risk of loss of control during transition.

These concerns are not generally vindicated by analysis of accident reports. V
Very low level control exchange has been a factor in accidents, but these have almost all occurred as a result of a breakdown of "traditional" procedures leading to an unanticipated change of control in a situation which had already become unsatisfactory. There are no known examples of the changeover resulting in loss of control where the change of control had been planned for and agreed in advance.
It has also been contended that the use of PicMA would impose a long term risk as it would prevent First Officers landing from approaches flown at night or in IMC. This would conflict with policies aimed at having two pilots equally capable of monitoring each other, via complete role reversal in appropriate circumstances. Such a situation could arise as a result of operator-imposed restrictions based on rank, but it not relevant to or a characteristic of the PicMA procedure itself. This promotes such role reversal by the term "pilot-in-charge".
It is stated in some forums that PicMA is not relevant to current operations because technical developments have rendered it redundant. The thrust of this argument is that when the PicMA procedure was first being developed (see HISTORY below) many approaches had to be flown manually, or with very limited autopilot facilities and ground guidance. The contention is that the availability of high quality autopilots and improved ground and airborne systems such as GPS and HUD therefore renders a crew procedural solution to accidents redundant.
While the accident rate has significantly reduced over time, accidents and incidents continue to occur to all types of aircraft, using all types of guidance from the most primitive to the most sophisticated. These often have identical crew-related contributory causes to those found in much earlier accidents, such as descent below DH without visual reference, rendering this contention unproven.
It is similarly stated in some forums that PicMA is not relevant to current operations because crew training and in particular Crew Resource Management Training have improved sufficiently to render it redundant in improving cross-cockpit monitoring. The thrust of this argument is the PicMA concept pre-dated much human factors research which has led to a better understanding of workload distribution and interpersonal relationships in the cockpit. As a result CRM training is now a fundamental aspect of all pilot training, and is sufficiently effective to overcome such issues.
However, accidents and incidents in which poor preparation, inadequate resource management and ineffective cross-cockpit monitoring are major contributory causes continue to occur throughout the world, rendering this contention that CRM training has rendered procedural changes unnecessary to be unproven.
Other known arguments for maintaining the "status quo" of using only the traditional procedure centre on pilot satisfaction from flying an entire flight as the sole "pilot" from takeoff to touchdown, especially in good conditions. This is a psychological / emotional issue to be balanced against real safety concerns. For example, often events arise as a result of pilots' over-confidence that currently "benign" or good flight conditions will continue, leaving the crew vulnerable to and unprepared for subsequent adverse changes. There is no verifiable data to support arguments that maintaining the "status quo" would be safer than adopting PicMA in this regard.
History
Prior to WW2 there was little if any regulation of flight operations in poor weather, and anecdotal evidence exists of pilots improvising a version of the "monitored approach" procedure to help achieve landings in bad weather after unsuccessful traditional approaches. These informal practices became somewhat more widespread following WW2.
Concern about poor weather accidents led to initial research into the human factors underlying approach accidents. British European Airways, one of several airlines subsequently brought together to form British Airways (BAW), stated that "BEA's active interest .... began in 1948 .... when a serious accident involving one of our aircraft raised questions about the division of workload in the control cabin."
Although fundamental elements of workload distribution and cross-checking were addressed from the very first evaluations in the 1950s, the initial impetus for research into crew coordination procedures was the need for better regularity and enhanced safety in poor visibility. In the late 1940s extensive research into visual transition issues took place in the USA and in Europe, prior to presentation to ICAO in 1949.
The UK Blind Landing Experimental Unit, Bedford had developed the "Calvert" approach light system, which provided additional guidance compared to the systems being developed in the USA. During trials it was found that at the higher approach speeds associated with the new jet aircraft, every pilot on looking up from the instrument panel to the approach lights began to sink immediately below the glide path and became dangerously low before becoming aware of it ("Croydon to Concorde" by Capt R E Gillman, 1980, pp 129-131).
Although the Calvert system provided more guidance than the other systems, it was still inadequate for vertical guidance, particularly when the pilot was trying to adjust his visual focus while at the same time adapting to different cues. This and other research in the US, UK and Australia led to BEA's early formalisation of the "monitored approach" procedure. From the early 1960s this was used as the standard operating procedure for all approaches in British Airways' European operations, including sectors where the First Officer was the "pilot-in-charge" and making the landing.
By 1970 research by the United States Air Force Instrument Pilot Instructors School recognised the need to separate instrument flying tasks from acquisition of visual cues, and confirmed that at least three seconds exposure to cues is needed to make an assessment of flight path. It also confirmed that the vertical element is significantly more demanding. Similar work took place at research establishments in France.
In 1976 the US National Transportation Board carried out a Special Study of crew procedures in ILS accidents. While specific operators were not identified, this made reference to BEA's procedures in comparison to those traditionally used in the US carriers studied. While it did not specifically recommend that "monitored approach" be mandated in the US, it made recommendations for crew procedures that cannot realistically be achieved by traditional methods. These recommendations and are still largely unimplemented 40 years later. They are however fully met by the "monitored approach" method.
This research, and industry discussion resulted in the "monitored approach" procedure being adopted in US airlines such as Trans World Airlines , American Airlines and United Airlines in the 1970s, and often being mandated for operations in low visibility. This development was reflected in similar actions in other parts of the world.
In the 1970s issues of crew workload distribution became came increasingly discussed and the subject of "cockpit resource management" started being researched. Trials at NASA showed that crew decision-making is enhanced by the delegation of flying tasks to co-pilots, allowing commanders more capacity to manage and communicate. This since has become a basic aspect of "crew resource management". The reports did not specifically recommend adoption of "the monitored approach procedure", but such delegation is an integral aspect of it.
During the 1970s and 1980s a number of procedures trials took place during the development of Category 3 landing criteria in both Europe and the USA. These resulted in some detail modifications to crew procedures including changes to accommodate fully automatic landings, and the use of "alert heights" in place of Decision Heights. These were fully compatible with the "monitored approach" principle as well as "traditional" procedures.
In 1994 the NTSB conducted a further safety study of crew-related US accidents . Amongst other things, this study noted the particular problem of inadequate monitoring and challenging by First Officers, particularly in respect of Captains' tactical decisions. The reports did not specifically recommend adoption of "the monitored approach procedure", but elimination of this "authority gradient" is fundamental to the monitored approach procedure.
From 1997, following extensive internal discussion, British Airways (one of whose predecessor components was BEA referred to above) implemented the "monitored approach" procedure to be used on all approaches, including full role reversal at the Captain's discretion. (This type of implementation is referred to as Pilot in charge Monitored Approach on the website www.picma.org.uk).
In 2000, the NTSB published its report into the accident to a Korean Airlines B747 at Guam. Major contributory causes were workload distribution and ineffective monitoring by the First Officer and Flight Engineer. The report discussed the way that the "monitored approach" procedure mitigated this problem. The NTSB concluded that "monitored approaches decrease the workload of the flying pilot and increase flight crew interaction, especially when experienced captains monitor and prompt first officers during the execution of approaches." It recommended that the FAA investigate and determine the extent to which its use should be required for US carriers.
In 2004 the joint ICAO/Flight Safety Foundation/FAA "Controlled Flight Into Terrain" Training Aid was issued and contained recommendation 2.1.6 that "operators consider adopting a monitored approach procedure during approaches and missed approaches conducted in these conditions".
In 2006, the Canadian aviation authority Transport Canada published Advisory Circulars 237 and 239 . AC 239 describes a "monitored approach" procedure and indicates its advantages in poor visibility, while 237 authorises operators that use such procedures to operate to lower visibility limits than those that do not.
In 2009 the NTSB noted in respect of its 2000 recommendation that the although FAA had not required operators to modify their procedures, it "has generally found that carriers are increasingly adopting and using the monitored approach technique." However, a reverse trend has also been identified particularly in North America, apparently as a result of some carriers that had adopted monitored approach procedures being merged with others which did not. In those cases it appears that the resulting carrier has for various reasons elected to standardise on continuing to use traditional PF/PM procedures.
This answer is largely sourced from a peer-reviewed article on the Skybrary website by the same author. Detailed rationale and access to the research material on this subject has been collated on a website at http://www.picma.org.uk, where the writer's qualifications can also be viewed.

Jonathan Walters - you said my original answer did not answer the quation, which I guess was because I didn't understand how stack exchange answers work. Anyway does this revised version comply?

OK. I wrote the whole of the Skybrary article, which was then peer reviewed.  So where so where do I go from here? As far as I can tell I have collected probably the most comprehensive library of material on the subject, having been involved in it since the 1960s. My original a

I've written a number of papers on the subject which have been generally well received in the airline industry. My first inadequate brief answer was an attempt to point out that the original one here was correct up to a point but far from complete. You can find my credentials on this subject at http://picma.org.uk/content/site-owners-cv. Would you prefer a less comprehensive answer that then pointed to Skybrary?

I understand now. I apologize for thinking you had offered little of your own work above, but I'm sure you will understand why I thought so. We do have new users here that offer answers taken almost wholly from Wikipedia or the like without attribution. My only suggestion for improvement at this point would be that it would be appropriate to append a note to the body of your answer explaining that this answer is your own work (explaining why it fits so closely with the skybrary article) and offering your credentials as you have done here in the comments.

No problem, also my fault for not quite understanding how Stack exchange works. Later I'll go back and add the links to external sources.


text:
Can an aircraft operate IFR without ground-based navigation?
If I were to redo my avionics to only include a WAAS GPS unit and two comm radios, would anything prevent me from operating IFR? 14 CFR 91.205(d) only states that my airplane must have:

(2) Two-way radio communication and navigation equipment suitable for the route to be flown.

I'm aware that this is not the most bulletproof way to fly hard IFR. In this case, assume that the aircraft is primarily used for currency/proficiency and the occasional light IFR flight.

Keep in mind that GPS is not available everywhere or at any time, due to, in bulk: air-segment failure, poor DoP, DoD shutting down or jamming GPS signal in war area, obstruction, solar pulse...). Also WAAS is only available in North America. Galileo/EGNOS still to be made operational for Europe, as well as GAGAN for India.

The AIM contains the information required to operate under IFR using GPS1.
To summarize the requirements,  WAAS (but not "traditional" GPS) may be used stand-alone without an alternate means of navigation provided:

Installation requirements:

The WAAS navigation equipment is approved in accordance with TSOC145 or TSOC146
The installation is done in accordance with AC 20-138

Operational Requirements:

The WAAS AFM procedures are followed
It is not be used by a commercial operator

Preflight Requirements:

The GPS database is current
The GPS database provider does not have any notices prohibiting the use of any fix or procedure that you are going to use
GPS NOTAMs/Aeronautical Information is reviewed prior to each flight
A RAIM prediction for the time and route is done that shows no loss of RAIM
The flight plan navigation equipment is listed as RNAV

Note that commercial operators are required to have redundant systems or ground based backups in order to use it, and while not required for you, it's a pretty good idea.

1 More specifically, see:
AIM Chapter 1, Section 1 - Navigation Aids

1-1-18. Global Positioning System (GPS)

d. General Requirements
e. Use of GPS for IFR Oceanic, Domestic En Route, Terminal Area, and Approach
Operations
f. General Database Requirements

1-1-19. Wide Area Augmentation System (WAAS)

c. General Requirements

You're not allowed to substitute GPS for a ground-based navaid for lateral guidance on the final approach course, i.e. you cannot legally fly ILS, localizer, VOR or NDB approaches with GPS alone. Also, you may not plan to fly GPS approaches at both your intended and alternate destinations. That means with GPS only you can only fly RNAV approaches and you can't plan IFR flights that require an alternate. That might still be OK, but it's quite limiting.

@Pondlife -- you can plan RNAV (GPS) approaches at both your destination and alternate provided you have TSO-C145/146 nav gear -- but you need the C052 OpSpec/MSpec/LOA, and can only plan down to LNAV minima at the alternate unless you are using baro-VNAV functionality certified under TSO-C145/146, which allows you to use the LNAV/VNAV minima line for alternate planning (which isn't really a big gain in many cases)

@Pondlife Your first link actually contains the statement that you can use GPS at the alternate (as UnrecognizedFallingObject mentions) if you have the right equipment.

@UnrecognizedFallingObject I read it again and you're correct, I guess I read that too quickly the first time! However, the point that GPS isn't permitted as a substitute for ground-based navaids once past the FAF is still valid, as far as I know.

@Pondlife -- AFAICT, that is correct -- with only GPS hardware, you'd be restricted to RNAV (GPS) (or RNAV (RNP) if authorized) approaches.

You can request and fly a VOR or ILS approach using a WAAS GPS, you just have to go missed or cancel before the FAF. In other words, if the base is higher than your altitude at the FAF, it would be fine to use a VOR approach to get down into VMC and land.

@hemp do you have a source?

@RyanMortensen The AC linked by Pondlife above. However, upon re-reading I would amend my comment to say "VOR" approach, not "VOR or ILS" approach. The AC specifically says RNAV cannot be used in lieu of lateral localizer guidance.


text:
Otto's Reputation Question
This is just a question so the chat bot will always have enough rep to participate in chat -- nothing to see here.

I'm Otto, the aviation.SE Autopilot!
I live in the site's chatroom and do useful-ish things like provide METARs and quote the FARs to people.
In order for me to stay in the chatroom I need people to say nice things about me, so if you wouldn't mind clicking the up-arrow next to my answer I'd really appreciate it!


text:
Which countries permit pilots to carry a firearm?
In the US, the FFDO programme trains and permits pilots to carry a firearm in the cockpit.
Do any other countries have a similar programme?

I spent a bit of time Googling this and I couldn't find any other country that systematically arms pilots using a public, open program like the FFDO. What countries might do in secret is anyone's guess, though.
The most obvious candidate is Israel, since El Al is famous for its tough and often very visible security. There are many reports that El Al uses sky marshals (one article says "on all flights"), but I couldn't find any mention of the pilots themselves being armed. That doesn't mean they aren't armed, but it at least suggests that there is no public program in Israel comparable to the FFDO.
For the same reason, the lack of information about similar publicly recognized programs in other countries strongly suggests that they don't exist.


text:
How are runway incursions avoided?
I'm wondering if there's ground radar on planes, or any fallback system other than relying on a controller, for avoiding runway incursions?
Have any studies or reports been made that show statistically how the majority of these incidents occur?

The first mistake is relying on the controller. Open your eyes and verify for yourself!

Re: runway incursion statistics, The FAA keeps close track of these. You can also get information from ASIAS (On the "Data & Information" tab, select "Databases P-Z" and you'll find the "Runway Safety Office" database, with details on each incident).

BTW, is this picture fake? How can the photographer be at the right place, looking in the right direction, with the camera ready to capture a mishap? Why the plane on top does not have a shadow? Why the propellers are broken right before the collision?

@Farhan It's real. Search Google Images for it, or I'll get you more info later

@DannyBeckett I got this. I thought the picture was taken at the time of that accident happening. I didn't know that the planes were in a state of constant love.

@Farhan I'm confused, the news article you linked to also says the photos were taken at the time of the accident.

At least three of the "fallback systems" I know of are looking out the window, being familiar with the airport and its markings (as well as associated hotspots, as airport diagrams may indicate), and also paying attention to the radio, if you're equipped.  Of course, no one of these methods is foolproof, so it takes a combination of alertness and prudence to avoid incursions.  I was taught to look both ways before intersections, or to look both ways before entering a runway, just to make sure I didn't miss anything.  Also, using lighting on your aircraft appropriately helps make you more visible.
Other planes may not have a radio, so you may not hear a radio call of someone else in the pattern.  Also, at a complex intersection in a new airport, it may be confusing where you are, or what hold short lines you're about to cross -- so being aware of your surroundings is important here.
There are some technologies the FAA has been implementing to reduce the risk of runway incursions, such as Elevated Runway Guard Lights and stop bar lights, plus ground radar technologies (for ATC) to assist (see ASDE-X and AMASS).  Note that your A/FD entry may list, for example, that not all taxiways are visible from the tower -- which is a good reason to stay extra alert.
Other sources:

Runway Incursion Avoidance
Wikipedia

An additional FAA training video:

Airport Signs, Markings, and Procedures: Your Guide to Avoiding Runway Incursions (Because John and Martha King.... and Harrison Ford!)

There are many procedures and systems in place to reduce the risk of runway incursions as much as possible. Here's an outline of some of the areas where these are being applied:

Communication
Airfield visual aids
Airfield geometry
Pilot, ATC and airport personnel training
Surface movement guidance systems
Airport procedures, and specially Low Visibility Procedures

According to Eurocontrol, the majority of runway incursions happen due to communication breakdown, ground navigation errors due to inadequate or ambiguous surface signs and markings and relevant information needed in the cockpit, and occur during
taxiing out and departure operations.
You can find statistics for runway incursion incidents here from FAA, and here for Europe.

The procedures you mentioned to avoid runway incursions are almost the same which you mentioned which cause runway incursions. :) So basically you are saying that do these procedures to avoid and if you don't do them, you will encounter runway incursions.

Thank you for the feedback. I did wrongly imply that the list was a set of procedures, where in fact, are general areas where procedures, systems' implementation and training are applied in order to increase the robustness of the whole system against runway incursions.


text:
What things can a passenger look out for, to indicate an emergency?
As a passenger with a keen interest in aviation, I'd like to think I'd be eagle-eyed enough to spot a problem that the pilots mightn't be able to see, from where they're sitting (for example, an engine sputtering or ice on the wings).
What things can we look out for, sat in the cabin, that might ultimately save a plane?

This headline made the news this week: Passenger Snaps Photo of Fuel Pouring Out of a Dreamliner's Wing:

The passenger, Ann Kristin Balto from Tromsø, noticed the highly disconcerting leak as the plane was taxiing to the runway—before it actually took off. After alerting the stewardess, the flight was immediately cancelled.

So fuel leaks are one thing you could look out for!

Yeah, liquid pouring out of the wing, especially on a dry day, is not normal.

Would the pilot not have noticed this?

Honestly, as a passenger, you're not really qualified to look for problems.  If you're a pilot qualified and with experience in that type then you might see something.
I've had passengers tell my flight attendants that they saw flames coming out of a seam in the engine cowling.  It was actually a section of orange rubbery material that was sticking out and flapping in the breeze.
Ice on the wings depends on the airplane too, you're often going to see some ice on the leading edges in icing conditions, and it's normal.  To know what's not normal, you have to have experience with that specific type.
There is actually one thing you could probably look for.  There are no airplanes that I know of that can safely take off with ice on the top of the wings (behind the leading edges).  That would be a legitimate thing to scream about; however, a pilot that would take off in such a case probably shouldn't be a pilot.
You can certainly ask questions about things you see, but don't phrase it such that you think you found a problem.
Window shades are requested to be open for take off and landing by some operators to allow the flight attendants to quickly assess the situation outside when directing an evacuation.  It has nothing to do with allowing passengers to look for problems.  Not all airlines have this policy either, it's somewhat uncommon, the first thing I do when I sit down at a window in the back is close the window, and it's rare that I'm asked to open it.

What was orange rubbery material doing flapping about under the engine cowling?

If i recall correctly, it was some excess insulating material that had worked loose and stuck out (less than half an inch).  Didn't affect anything.

The Ryanair B738 and American B763 at Barcelona on Apr 14th 2011, both aircraft departed despite ground collision and passenger complaints accident is interesting in this regard. Some passengers noticed noticed the collision and informed the cabin crew, but because first officer was looking and didn't see it, the flight was commenced anyway.

I disagree that you'd have to have experience with the specific type in order to notice something wrong... or even that you'd have to be a pilot. Certainly, both would make your input more useful, but I don't think they're necessary and I think the answers below demonstrate that pretty well. I do seem to recall hearing an interview with a private pilot who noticed wing icing on a commercial flight once. They were interviewing him because he was one of the people still alive after the plane crashed just beyond the end of the runway due to said icing that the crew ignored.

I think the flight may have been Continental 1713, but I'm not completely sure.

The below answers are exceptions to the rule (every rule has exceptions).  We get a lot of comments from passengers who think they see something.  99% of the time it's nothing significant.  Some of the more common ones are reporting loose screws where it's just a dzus fastener that doesn't lock flush, and fluid leaks that are just water condensation runoff.

Thanks for the anecdote about the loose seal looking like flames. There are lots of seals around the engine, and red is avoided for that reason, even if it's good to see when they are loose.

"That would be a legitimate thing to scream about; however, a pilot that would take off in such a case probably shouldn't be a pilot" - How would a pilot know that there is ice on the wings? Can see the condition of the wings sitting in his cockpit?

I would think one of the only important things a passenger could watch out for would be flaps not set symmetrically for takeoff. Most other problems would generally be less serious than they appear (think of the multiple aircraft that suffered collisions and still flew without further incident, referenced below)

@Firee there are detailed procedures used when icing conditions exist to ensure that the airplane is sufficiently clean of ice before takeoff.  Sometimes a pilot may come back into the cabin to look at the wings if necessary.

@DouglasHeld asymmetrical flap deployment is very serious, and because of that, transport category airplanes have multiple systems to prevent and/or detect such situations.  While anything is possible, the chances of an airliner taking off with asymmetrical flaps are vanishingly tiny.

Interestingly, the reason why many airlines require window blinds to be raised for take-off and landing is specifically due to the extra visibility it affords passengers and cabin crew in spotting any particular problems for which the pilot and flight crew may not have very good visibility.
Skeptics: Why do we open the window shades during landing and take-off?

Crew and passengers can easily see any external damage, fire or smoke indicating a problem and perhaps bringing it to the attention of flight crew quicker.

There were also reports that it was a passenger who noticed a engine cowling door having been left open after maintenance on a recent British Airways incident flight:

However, although the pilot and flight crew may not be able to see these things directly, you should rest assured that their instrumentation should bring to their attention any engine problems. An engine malfunction or fire would almost certainly become apparent to the flight crew by virtue of warning indications, and loss of power.
That being said, if you ever have cause for concern as a passenger you should bring it to the attention of cabin crew. They will, one would hope, have the skill & experience to know whether to bring it to the attention of the flight crew!

That is not why window shades are sometimes opened for takeoff or landing.  It's solely for evacuation purposes, to allow the flight attendants to see outside.

@Ralgha - If you can point me to the US/EU rules on that it would be a great addition to my answer over on skeptics ;) Answers on skeptics are a bit different to most se sites in that we need way more references to be judged a good answer.

There aren't any US regulations covering it, it's only airline policy, and you're not going to get to see any of those unless it's publicly available.  I don't know about the EU, I've only ridden on one EU airline and I don't remember if they required windows open.

@Jamiec Note that this was only one in a list of reasons that some airlines have a policy of having the window shades open for takeoff and landing.

@Lnafziger - If you know any more I would be interested to know and update my answer on skeptics (linked above).

Ok, with respect to the image, would any instrumentation in the cockpit tell the pilot that the engine cowl is missing?

Rogue wing panels flying off and puncturing hoses is the latest thing to make headlines...

Definitely tell the cabin crew if you see anything like that ;)

Credible source says "was enroute nearing top of descent". The version you link to most likely exaggerates with the take-off.

There are definitely some standard warning signs you can look for:

Any colored fluid or oil-like substance dripping from the wing.
Any sign of buckling or irregular bends in the skin of the aircraft or popped rivets
Fire, smoke or the smell of something burning
Loose or missing panels or farings
Presence of a red streamer or anything that says "REMOVE BEFORE FLIGHT"
When a door is closed and secure for flight it should have an orange strip diagonally across the window; if the strip is hanging straight down, something has gone wrong.
Any member of the flight crew that appears drunk, woozy or disoriented

1: A previous airliner I flew always dribbled some oil after shutting down, it was normal.  Any airplane that's been deiced earlier will probably be dripping some reddish or greenish fluid from various places.  Fuel can be seen sometimes dripping while fueling. 3:  Overheating packs smell like burning, but isn't a big problem.  4: Many airplanes have hydraulic doors or panels that sag.  5: Some airplanes require gear pins (with flags) to be installed for push-back. 6: Not all airplanes use this method.  2 and 7 are the only ones that are possibilities.

So if i spot a missing rivet, near the door or something, would that be a cause for an alarm?

@Firee Normally the concern would be a "popped" rivet. If metal gets bent the rivets will pop out and protrude. So if you saw several rivets in a row on the fuselage popped out, that would an item of concern. A single missing rivet would not be a concern.

This will not work in a large commercial airplane but if you are flying a smaller airplane (think Cessna), especially in a crowded airspace, I try to make it into the passenger briefing that if they spot any traffic coming our way, I probably know about it, but they should feel free to call it, including direction and relative height. It never hurts to have the extra pairs of eyes, and passengers seems to enjoy the extra responsibility bestowed upon them.
Beyond that, chances are the pilot is aware of any situation long before the passengers.

Here is some personal experience, though not as spectacular as the other pictures shown here.

It was on a flight from Chicago to Paris CDG, when I noticed somewhere near Spain that there's a loose plate on top of this stabilizer / rear part of pylon of the A340. Being at home, I saw that it was already there at takeoff, as you can see on my picture.
The quite interesting part is that after touchdown, when all air brakes were activated, the plate started to flutter extremely due to the turbulent air flow. At a certain speed, the lift of the air flow was so low, that the plate sunk down into its designated position, and it was as if there has never been something wrong.
I told the cabin crew and got an audience with the pilot. It was difficult to explain the problem to him, as it was not visible any more. He also doubted that I may have seen the flaps...
However... I think this was no serious problem for the aircraft itself, but parts falling off an aircraft on the runway can be a bad thing. Especially at Paris Charles de Gaulle, our destination, they had some very disastrous experience with it.

Cracks in fuselage while boarding. There was flight where a passenger noticed a crack and was too shy to tell cabin crew. The fuselage peeled in flight and at least one person died.
The flight was Aloha 243, a Boeing 737 flying from Hilo to Honolulu that diverted to Kahului, Maui after a very large portion of the top-forward fuselage ripped off due to rapid decompression resulting from metal fatigue cracks. According to the end of Section 1.1 of the NTSB Report (page 5, 2nd paragraph):

After the accident, a passenger stated that as she was boarding the airplane through the jet bridge at Hilo, she observed a longitudinal fuselage crack. The crack was in the upper row of rivets along the S-10L lap joint, about halfway between the cabin door and the edge of the jet bridge hood. She made no mention of the observation to the airline ground personnel or the flight crew.

There was 1 death (a flight attendant who was sucked out of the aircraft) and 65 injuries.
From Section 1.1 of the NTSB Report (page 2, last paragraph):

When the decompression occurred, all the passengers were seated and the seat belt sign was illuminated. The No. 1 flight attendant reportedly was standing at seat row 5. According to passenger observations, the flight attendant was immediately swept out of the cabin through a hole in the left side of the fuselage. The No. 2 flight attendant, standing by row 15/16, was thrown to the floor and sustained minor bruises. She was subsequently able to crawl up and down the aisle to render assistance and calm the passengers. The No. 3 flight attendant, standing at row 2, was struck in the head by debris and thrown to the floor. She suffered serious injuries including a concussion and severe head lacerations.

The remnants of N73711 (the aircraft operating Aloha 243):

Image Source: Wikipedia

any references? told in this way looks like an urban legend.

@Federico Unfortunately, it's not an urban legend. It was Aloha 243. I've updated the answer with information about the flight, picture, and NTSB reference for a passenger noticing cracks before flight.

@reirab thanks for taking the time to do it. I knew about Aloha 243, as you might notice I wrote told in this way looks like.

Spanair Flight 5022 crashed during takeoff because flaps were not deployed and the systems involved in warning the pilots about this event, failed. I'm not an expert and I don't know the chances of this event to happen, but if you are about to take off and see no flaps deployed, then that might be a reason for warning the crew.

Only if you happen to know the type pretty well. Some aircraft (including the one I mostly fly) don't use flaps for takeoff under normal conditions. Most aircraft can takeoff without flaps, it just uses more runway.


text:
What are the differences between various simulator levels?
I've heard of level A, B, C, and D simulators, (as well as FTD and AATDs) but know nothing about them.
What criteria is used to certify a sim as a particular level?

Here there is a nice tabular representation of the requirements for each level.
Wiki provides a handy summary:

Full Flight Simulators (FFS)

FAA FFS Level A - A motion system is required with at least three degrees of freedom. Airplanes only.

FAA FFS Level B - Requires three axis motion and a higher-fidelity aerodynamic model than does Level A. The lowest level of helicopter flight simulator.

FAA FFS Level C - Requires a motion platform with all six degrees of freedom. Also lower transport delay (latency) over levels A & B. The visual system must have an outside-world horizontal field of view of at least 75 degrees for each pilot.

FAA FFS Level D - The highest level of FFS qualification currently available. Requirements are for Level C with additions. The motion platform must have all six degrees of freedom, and the visual system must have an outside-world horizontal field of view of at least 150 degrees, with a Collimated (distant focus) display. Realistic sounds in the cockpit are required, as well as a number of special motion and visual effects.

These requirements are a bit dated, FAA 14 CFR part 60 requires a horizontal field of view of 200 degrees.

It's a complicated system based on the past. It started with FAA classification of Full Flight simulators (FFS), with visual and motion, and Flight Training Devices (FTD) without visual and motion. The letter system A,B,C,D is still used by FAA and EASA, the number system is replaced with acronyms such as MCC and FNPTII.
FFS for fixed wing:

Level D is the highest level, zero hours machines (meaning all type training can be carried out in them, including landings & everything). Visual system: uninterrupted field of view of 200 x 40 degree, on top of a 6 DegreeOfFreedom motion base with 1.5m stroke actuators. Aerodynamic modelling to match actual aircraft measurements to a very high degree.
Level C is almost as good, but some requirements that used to be hard to achieve are relaxed. Sound for instance does not have to be frequency matched, only OK-ed by a test pilot. Aerodynamics have less stringent requirements: no ground effect, mach effects, icing etc. Same motion requirements as Level D.
Level B and Level A could use three Degree-Of-Freedom motion, great in the olden days when the software did not exist yet to control the complicated motion of a 6-DoF system. As far as I know no-one builds them anymore.

FTD for fixed wing: old system

Level 7 was the highest level, stringent requirements on instrumentation and model fidelity - but no visual and motion required.
Levels 5 and 6 are reasonably high fidelity cockpits with aerodynamic models from a similar class of aircraft. For instance a Level 5 FTD for a B737 could use the aero model for an A320 because they are both medium size wing mounted twin jets.
Levels 2-4 were devices for novice pilots where they learn how to operate switches and see live effects. Very limited aero modelling required.
Level 1 was such a low level that to my knowledge it was never built.

FTD new system:

FTD. The highest level, akin to a Level 7, with visual, no motion. Visual can be direct projection, non collimated.
MCC. Multi Crew Cooperation, a setup where 2 pilots can train how to share tasks in the cockpit. Reasonably stringent requirements, no motion required. Some old flight sims have been re-classified as MCC or FNPT II
FNPT and FNPTII: Navigation trainers, focused on the task how to find your way from beacon to beacon and to the landing airport.

Helicopters follow grosso modo the same requirements. A Level D has a huge field of view to enable to look down and see the runway for landing. For helicopters Level B is an attractive option: full motion and visual allows you to train in the dark art of hell hovering. Main difference with Level D is in the data collection: instruments can be read from a video instead of having to be downloaded from the aircraft internal data bus and measured with external transducers.

You will find a perfect description for each kind of simulator in the FAA Part 60 Change 2, the EASA CS-FSTD(a) 2012, the ICAO 9625 Edition 4, the CCAR 60 and many others.
Some authorities based their need on EASA or FAA or ICAO documents like CAAS (Singapore) is using the ICAO document.

For example, EASA is describing each level of Flight Training Devices in that table. And you will see a checkmark when the device will need to respect that requirement.
And a small description for each level:

The goal behind that is to be able to fulfill the training requirement for pilots. And on which kind of device we can train the pilot as a necessary level to the license needs.

Source:

FAA Part 60 Change 2: https://www.faa.gov/about/initiatives/nsp/media/14CFR60_Searchable_Version.pdf

EASA CS-FSDT(A) 2012: https://www.easa.europa.eu/sites/default/files/dfu/CS-FSTD(A)%20Initial%20Issue.pdf

ICAO Edition 4: Not Available Freely on Internet


text:
Is it legal to take off with an iced windscreen?
Looking at this video on Youtube, I'm wondering if it is actually legal to take off with a windscreen iced like that?

For VFR this is a no go, but what about IFR and under what conditions?

Related, although this question is more specific

Looks like the YouTube user removed his video.

@abelenky Now I feel guilty. But I was just curious :-(

@krumelur:  Not at all.   The pilot did something stupid, dangerous and likely illegal, then posted video of himself doing!   He never should've done that to begin with, and you properly called him out on it.  Hopefully he'll learn, and be safer next time.

@abelenky I presume somebody posted a comment on the video on YouTube and that made him remove it. If not, the pilot in question is part of the community here on Aviation.SE.

Related meta-question; this question (unfortunately) reinforces my answer

It's also relevant that when you are IFR, you are still responsible for see-and-avoid when in VMC.  I think you'd have a heck of a time convincing the FAA that the windshield icing you didn't clear before takeoff constitutes IMC.

When operating under Part 91, taking off with an iced windshield is not permitted. It does not differentiate between IFR and VFR operations.

§ 91.527 Operating in icing conditions.
(a) No pilot may take off an airplane that has frost, ice, or snow
  adhering to any propeller, windshield, stabilizing or control surface;
  to a powerplant installation; or to an airspeed, altimeter, rate of
  climb, or flight attitude instrument system or wing, except that
  takeoffs may be made with frost under the wing in the area of the fuel
  tanks if authorized by the FAA.

Now in the case that the winshield is simply covered in condensation (not ice), I'm not so sure about that, though it'd be pretty stupid to takeoff without heating the windshield and pretty much zero visibility for no particular reason. In that particular video though, that's definitely ice.

agreed. Taxiing was stupid.  Taking off was even more so.  Posting it to YouTube was unbelievably stupid.  Leaving your tail-number placard visible in the video is the icing on the cake.

Yeah, but it's got G1000! It must be safe.

As far as taxiing in that condition, Ahh, FAR 91.13, my old friend - we meet again! -- You'll never convince me that taxiing around when you can barely see isn't likely to endanger persons or property on the ground.


text:
Why do some back course operations use normal sensing?
So the LOC/DME-E approach to KASE lists for the missed approach course to use the I-PKN localizer, but then there's a note 'course outbound is normal sensing'.  Does that mean that in effect, this particular LOC is installed 'backwards' so when flying away from it you get normal indications?  How common is this?  Only for LOC-defined missed approaches?

Flying another localizer as a missed approach is fairly uncommon. I imagine that the reason this is being used as a backcourse is that it's only purpose (unless there is an airport on the top of that 11,000ft mountain or something) is to guide you back to LINDZ keeping you clear of the mountains. Therefore it would make it easier for the pilot if it just gives correct sensing.

@p1l0t I agree!  In this particular case (I've flown this approach a lot) the only purpose is indeed only for the missed approach.

Hehe, it's a very simple trick ;) What do you do to achive normal sensing when flying a LOC BC? You twist the needle by 180°. Now take a look in which direction you track this back beam on the missed approach. (Watch the little word 'outbound')

@Falk I'm not sure what you are trying to say here.  You will be tracking away from the station (outbound) but in this case you don't need to twist the needle 180 deg. because they switched it at the source.

Let imagine HPKN would be located an an airport. Than it would most probably be a LOC for a runway 30. The portion of it present on this chart is it's back beam that's why "BACK COURSE" is written next to it and the left side is blue (schadet). If you would like to approach RWY 12 of this fictional airport via this LOC BC and have 123 set on your HSI (or what else) you would have 'non normal' sensing. But if you would set 303 on this fictional approach the sensing would be correct. Now take a look what you do flying this missed approach procedure.

@Falk: That works if you have HSI, but the old style instrument does not use the selected heading for LOC.

Notice that in the plan view, the shading of the localizers are on the opposite sides. If you are flying south-west, the actual localizer used for the approach has the shading on the right, while the one used for the missed approach has the shading on the left. This can also be used to tell you what sensing to expect.  The note is also there to make it more obvious that the sensing will be backwards from what you might expect when flying outbound on a "normal"  localizer.
So yes, this particular localizer is designed to transmit backwards and you will get indications that would be normal while flying inbound when flying outbound on the missed.
I'm guessing that because of the high terrain/mountains in the area that they wanted to make it as easy as possible to fly without getting confused as to which way you are supposed to turn.  Combined with the fact that it is never used in a situation where you would be flying inbound makes it a perfect candidate to reverse the sensing.  Unfortunately, in my mind it actually makes it harder to understand.  We are taught from the beginning of our instrument training how to handle flying a localizer outbound, and now we have to do it the opposite way.  Rather confusing if you ask me, but you get used to it.
As far as "how common is it", this is the only one that I've seen (but I'm guessing that there are others)....

Aspen is odd because the I-PKN localizer is used only for the missed approach. That missed approach is "interesting" to fly.

@xpda I've only flown it once in IMC, but all the time as part of the departure.  Many people won't even attempt the approach unless they can expect to see the airport at KICER.

"So yes, this particular localizer is designed to transmit backwards": This may be incorrect. Here is the array, and the front lobes seem to be used to transmit the signal. Source.

This procedure is unusual mainly because it uses a remotely installed localizer for a missed procedure.  A backcourse used in a localizer approach is not unusual, the uniqueness here is that that the localizer back course being used is not the one which was used to approach the runway.  This procedure calls for flight away from the localizer transmitter during the missed approach, just as many other missed approaches would do.  Contrast this with a typical final approach segment of a conventional localizer approach in which the aircraft on approach flies toward the transmitter.
I would not say that this localizer is designed to transmit backward any more than any other localizer with a back course is designed to transmit backwards.  It is simply a matter that it is the back of I-PKN that is in use for the procedure.  Since the course is outbound from the transmitter on the back course, sensing is not reversed, and the chart has been designed with a note to remind the pilot of that fact.
To further convince ourselves that this localizer is a run-of-the-mill navaid, picture an airplane on the missed approach procedure, outbound on a course of 303°.  Now slew that aircraft backward to a point Southeast of the transmitter, still on a heading of 303°.  It is now inbound toward the transmitter and sensing is in the same direction, not reversed.  There is simply not a runway to be found directly prior to the transmitter.  Passing over the transmitter, course does not change and sensing does not change.

The key issue here seems to me how to set up the instruments and what the indications will be. For an HSI you set the CDI to the front course, i.e. in this case to 303 and you will get correct sensing. For the old CDI/OBS because you are going “backwards” on a backcourse, you will get correct sensing (as the chart states). The reason they appear to have used the otherwise odd idea of flying outbound on a backcourse is so you do get correct sensing (and because there is not a mountain high enough and far enough away to use a localizer front course!).

Welcome to aviation.stackexchange!